Tires generally comprise a composite of rubber compositions with fiber reinforcements. In some instances, the fiber reinforcements are based on synthetic polymers, such as polyester. Polyester fibers have been widely used as a reinforcing material for rubber in tires. However, achieving good adhesion of polyester tire cords to rubber compounds has always been a challenge, primarily due to the high molecular weight, high degree of crystallinity and highly drawn characteristics of the final polyester fibers which leave only few free hydroxyl and carboxyl groups on the surface of the fibers. With a limited number of functional groups remaining on the fiber surface, the reactivity of the polyester fiber surface towards adhesive agents is therefore, very limited.
In the tire industry, two conventional adhesive systems are widely used for achieving good bonding between tire cords and rubber compounds: 1) the Resorcinol-Formaldehyde-Latex (RFL) method wherein an adhesive is applied to the tire cord and 2) the Hexamethylenetetramine-Resorcinol (HR) method wherein an adhesion promotion system is incorporated into the rubber compound. For greater adhesion effectiveness, HR systems often contain hydrated silica and, in that case, they are referred to as HRH systems. Detailed discussions of the conventional RFL and HR methods that are being widely used in the industry can be found in several comprehensive reviews, including J. P. Noe et. al., Rubber and Plastic News, 14, May 1978; T. S. Solomon, Rubber Chemistry and Technology, vol. 58, 561 (1985); and R. Iyengar, Rubber World, November 1987, p. 24–29. Extensive literature has dealt with achieving the desired adhesion levels for polyester/rubber composites; however, most approaches are directed towards the modifications of the RFL dips and/or the HR rubber systems to improve adhesion. In order to understand how each of these adhesive methods works, it is important to understand the basic process of forming tires and tire fabric.
In conventional tire fabric processes, yarn is shipped to a conversion mill where it is subject to the following process: a) yarn is twisted into a greige cord (an unfinished cord), b) the greige cords are generally woven into a unidirectional fabric stabilized with fine denier “pick threads” in the weft direction, c) an aqueous dip (commonly known as a resorcinol-formaldehyde-latex (RFL) adhesive system) is applied to the greige cord, d) the dipped cord is dried, and e) the dried cord is subjected to high energy treating step, often requiring relatively high temperatures (350–480° F.) for relatively long residence times (30–120 sec). The resulting treated cord is then shipped to a tire plant where it is formed (generally calendered or passed between heated steel rolls) into a cord reinforced rubber sheet which is ultimately built into tires. These steps are generally performed at a conversion mill whose location is separate from either the yarn production or the tire production sites. These separate production locations are necessary because the treating units tend to be high-volume, multi-story units capable of making fabric over 50 inches wide.
Typical tire “fabric” has at least 20–30 cords per inch, which adds up to over 1000 cord ends that are simultaneously treated. In instances where single-end cords are required, 100–200 greige cords are generally placed in a creel, then the individual cord ends are fed directly into a treating unit, and finally the treated cords are wound onto individual spools. Compared to conventional fabric treating operations, these single-end treating operations are very inefficient and hence much more costly.
When using the RFL method to treat tire cords, the cords are coated with an aqueous solution consisting of a mixture of a resorcinol-formaldehyde resin and typically a latex of a styrene-butadiene-vinyl pyridine terpolymer. The coating process is typically followed by high temperature heat treatment for drying and crosslinking the resin to form a strong network, which is necessary to achieve optimum adhesion to rubber. Due to the low polarity of the polyester fiber surface as discussed above, numerous modifications of the RFL method have been successfully developed and widely used in the industry to achieve good adhesion between polyester fibers and rubber compounds. These include: (1) applying an adhesive activated spin finish or overfinish onto the polyester fiber surface during the fiber spinning process prior to RFL treatment; (2) utilizing a two-step coating process where the greige cords are pre-coated with for example, a blocked isocyanate/polyepoxide mixture prior to the RFL coating; or (3) using a one-step coating process where the RFL coating is further activated with other adhesive systems. Typical adhesion active finishes are described in U.S. Pat. Nos. 4,348,517; 4,462,855; 4,557,967; and 5,547,755.
For example, U.S. Pat. No. 5,075,415 describes an adhesive composition that strongly bonds polyester fibers to rubbers with reduced adhesive deterioration at elevated temperatures. The one-step dip consists of adding a co-condensation resin derived from m-cresol, m-aminophenol and formaldehyde to the standard RFL mixture. U.S. Pat. No. 5,922,797 describes an RFL adhesive system where the adhesion was improved further through the incorporation of a fourth monomer in the latex polymer. More specifically, the adhesion of the epoxy surface activated polyester fibers to rubber was improved with an RFL coating containing a latex polymer derived from styrene, butadiene, vinyl pyridine and vinyl aldehyde monomer. U.S. Pat. No. 6,444,322 describes an adhesive composition for a textile-reinforced rubber product where the polyester or polyamide reinforcing fibers were pre-coated with a mixture of amine functional silanes and organosilanes with unsaturation capable of reacting with the rubber, followed by a standard RFL treatment.
With respect to the RFL method, it can be seen in the above-discussed literature, that the most common solutions for improving polyester tire cords to rubber adhesion involved combinations of the following systems: (a) surface treatment of polyester fibers with epoxy based finishes, (b) pre-coating the fiber with a blocked isocyanate/epoxy resin mixture prior to RFL treatment, and (c) mixing resorcinol-formaldehyde-latex coatings with other adhesion promoters. Most of these processes are followed by high energy treatments which are required to dry the latex dip, to pre-react the dried latex components and crosslink the RFL adhesive coating. Thus, both long time (>30 sec) and high temperature are usually needed.
The second method for achieving adhesion between tire cords and rubber is the HR method and, particularly the improved HRH version. As mentioned earlier, the term “HRH” refers to the three adhesive components that are typically compounded into the rubber: 1) hexamethylenetetramine or hexamethoxymethylmelamine (commonly referred to as formaldehyde or methylene donor), 2) resorcinol (or any other phenolic derivatives, commonly referred to as formaldehyde or methylene acceptor) and 3) hydrated silica. This method has potential as a direct bonding or dry rubber method. In this case, the tire cords are embedded directly into the rubber. The crosslinked resin resulting from the reaction between the methylene donor and methylene acceptor is formed in-situ during the rubber vulcanization process. The RFL coating step can be totally omitted. This method was originally developed for metal wires since these cannot be pretreated with an RFL system. The method was later successfully adapted to work with polyamide fibers due to its inherent high degree of polarity. This method has not yielded good adhesion to polyester cords.
HR modified stocks have been used to improve adhesion to polyester treated cords. U.S. Pat. No. 3,638,703 describes the use of different acids to complex with hexamethylenetetramine in the HR modified rubber system to prevent the strength loss via degradation of polyester fiber during rubber curing which is due to ammonia generated from the decomposition of hexamethylenetetramine. This reference mentions the use of greige PET fabric in rubber and the improvement in bonding PET to rubber when HR is present in the rubber. However, it does not (a) mention the importance of the activation of the PET or (b) provide any information regarding the ability to achieve the adhesion level necessary for use in tires. Instead it merely says that adhesion may be improved by using conventional cord adhesives. U.S. Pat. No. 3,738,948 teaches the use of metal soaps such as calcium stearate in HRH modified rubber stocks to prevent the degradation of polyester cords and consequently preserving fiber to rubber adhesion. U.S. Pat. No. 3,778,406 teaches a process of premixing finely divided silica with resorcinol as a method for improving adhesion of conventional polyester fiber to HRH modified rubbers. Incorporation of lead oxide was also found to improve adhesion further. It should be noted from this reference that the modified HRH rubber works well, provides good adhesion with greige Nylon fibers, but not with greige (“non-impregnated”) polyester fibers. U.S. Pat. Nos. 5,656,687 and 5,684,082 disclosed improved adhesion of polyamide, polyester and metal tire cords by incorporating in addition to methylene donor, methylene acceptor and silica, a maleated styrene-butylene-styrene triblock polymer. JP 06294072 (1994) describes a rubber reinforcing fiber material where the surface of the fiber is treated with a reactive blocked isocyanate-based polyurethane which can deblock at rubber cure temperature to generate isocyanate groups which can subsequently react with the adhesive components in the HRH modified rubber. However, the use of blocked isocyanate is not safe and not environmentally desirable. British Patent 1263915 describes a rubber cement compound for use in the production of reinforced rubber articles such as V-belts. The adhesion pretreated polyester yarn (DIOLEN™ 164S) was coated with a rubber cement prepared by mixing neoprene, fillers, oil, curing agents, resorcinol supported on silica and hexamethylenetetramine in toluene solution followed by drying.
In most instances, conventional processes that involve HR modified rubber compositions have problems with the generation of fumes associated with the use of pure resorcinol. The use of pure resorcinol also results in hydroscopicity. There have been many attempts to modify resorcinol to reduce this fuming and hydroscopicity. U.S. Pat. No. 4,889,891 discloses that resorcinolic resins derived from reactions of resorcinol with mono or polyunsaturated hydrocarbon compounds such as piperylene, dipentene, dicyclopentadiene or divinylbenzene can reduce fuming associated with pure resorcinol and improve rubber adhesion. U.S. Pat. No. 4,892,908 described benzoylresorcinol as an alternative replacement for resorcinol. U.S. Pat. No. 5,049,641 replaced resorcinol with styrene modified resorcinol-formaldehyde resin. U.S. Pat. Nos. 5,936,056 and 5,945,500 replaced resorcinol with resorcinolic or phenolic resins modified with epoxy resin, styrene or aromatic phenolic compounds. U.S. Pat. No. 5,206,289 taught a novel rubber stock composition modified with polyhydric phenoxy resin with improved cured rubber tensile strength at break. In most of these patents however, there was no mention of the type of fiber used and little to no adhesion data was reported in the examples.
Several patents that dealt with the problems inherent to the use of pure resorcinol also disclosed certain adhesive improvements in the materials. For example, U.S. Pat. No. 5,030,692 taught improving adhesion and mechanical properties of copper wire with resorcinolic novolac resins modified with cashew nut liquid or allyl and alkyl phenol. U.S. Pat. No. 5,021,522 disclosed the use of phenolic novolac resin modified with styrenic compounds to reduce fuming and hydroscopicity associated with pure resorcinol and to improve adhesion of copper wire. U.S. Pat. No. 5,244,725 disclosed the incorporation of hydroxyalkyl aryl ethers of di- and polyhydric phenols into HR modified rubber to improve adhesion of copper wire. U.S. Pat. No. 4,605,696 taught the use of phenolic esters such as resorcinol benzoate, resorcinol rosinate as a replacement for resorcinol in the HR rubber formulations to enhance adhesion of textile fibers or wire cords to rubber. U.S. Pat. No. 5,049,618 taught vulcanizable rubber compositions containing hydroxyl-aryl substituted monomaleimide as an alternative to resorcinol to increase adhesion of nylon and improve tear resistance of the resulting cured rubber stock. Although adhesion data was reported in these patents, the data dealt primarily with nylon cords and copper wires.
In a continuing effort to improve adhesion properties in rubber compositions for the tire industry, several patents describe using self-crosslinkable additives and other additives in the place of resorcinol. For example, U.S. Pat. No. 5,298,539 disclosed rubber compositions with improved tire cord adhesion using new self-crosslinkable additives. The additives include derivatives of melamine, aceto guanamine, benzoguanamine substituted with vinyl terminated radicals. U.S. Pat. No. 5,891,938 described the use of self-condensing high imino alkylated triazine resins for improved tire cord adhesion and reinforcement without the need for resorcinol or methylene acceptor. Only rheological cured properties of modified rubber stocks were provided.
With respect to the HR method, prior developments were focused on the combination of both the methylene donor and methylene acceptor in the rubber stock. One of the disadvantages of having both the methylene donor and methylene acceptor in the rubber is that they form a highly crosslinked resin during rubber curing, which increases the hardness of the final cured rubber and consequently leading to poor dynamic adhesion and fatigue resistance of the final reinforced rubber components.
Therefore, there is still a need in the tire manufacturing industry for a composition that has good dynamic adhesion, while ensuring that the final cured rubber composition is pliable and has good fatigue resistance. It is also important to ensure that the process, intermediate and final compositions are environmentally friendlier and safer than previously developed compositions and processes. Furthermore, there is still a need to provide methods for achieving improved adhesion between polyester cords and rubber over conventional methods previously mentioned and that can a) eliminate the use of an RFL coating, b) avoid the high temperature heat treatment required to cure the RFL adhesive resin prior to rubber curing, and/or c) reduce the hardness problem associated with the crosslinking of the bonding agents present in the HRH modified rubber stock.